Respiration - How it works

Photo by: Vanessa

Forms of Respiration

Respiration can be defined as the process by which an organism takes
in oxygen and releases carbon dioxide, one in which the circulating
medium of the organism (e.g., the blood) comes into contact with air
or dissolved gases. Either way, this means more or less the same thing
as breathing. In some cases, this meaning of the term is extended to
the transfer of oxygen from the lungs to the bloodstream and,
eventually, into cells or the release of carbon dioxide from cells
into the bloodstream and thence to the lungs, from whence it is
expelled to the environment. Sometimes a distinction is made between
external respiration, or an exchange of gases with the external
environment, and internal respiration, an exchange of gases between
the body's cells and the blood, in which the blood itself
"bathes" the cells with oxygen and receives carbon
dioxide to transfer to the environment.

This is just one meaning—albeit a more familiar one—of
the word respiration. Respiration also can mean cellular respiration,
a series of chemical reactions within cells whereby food is
"burned" in the presence of oxygen and converted into
carbon dioxide and water. This type of respiration is the reverse of
photosynthesis, the process by which plants convert dioxide and water,
with the aid of solar energy, into complex organic compounds known as
carbohydrates. (For more about carbohydrates and photosynthesis, see
Carbohydrates.)

How Gases Move Through the Body

Later in this essay, we discuss some of the ways in which various
life-forms breathe, but suffice it to say for the moment—hardly
a surprising revelation!—that the human lungs and respiratory
system are among the more complex mechanisms for breathing in the
animal world. In humans and other animals with relatively complex
breathing mechanisms (i.e., lungs or gills), oxygen passes through the
breathing apparatus, is absorbed by the bloodstream, and then is
converted into an unstable chemical compound (i.e., one that is broken
down easily) and carried to cells. When the compound reaches a cell,
it is broken down and releases its oxygen, which passes into the cell.

On the "return trip"—that is, the reverse
process, which we experience as exhalation—cells release carbon
dioxide into the bloodstream, where it is used to form another
unstable chemical compound. This compound is carried by the
bloodstream back to the gills or lungs, and, at the end of the
journey, it breaks down and releases the carbon dioxide to the
surrounding environment. Clearly, the one process is a mirror image of
the other, with the principal difference being the fact that oxygen is
the key chemical component in the intake process, while carbon dioxide
plays the same role in the process of outflow.

HEMOGLOBIN AND OTHER COMPOUNDS.

In humans the compound used to transport oxygen is known by the name
hemoglobin. Hemoglobin is an iron-containing protein in red blood
cells that is responsible for transporting oxygen to the tissues and
removing carbon dioxide from them. In the lungs, hemoglobin, known for
its deep red color, reacts with oxygen to form oxyhemoglobin.
Oxyhemoglobin travels through the bloodstream to cells, where it
breaks down to form hemoglobin and oxygen, and the oxygen then passes
into cells. On the return trip, hemoglobin combines with carbon
dioxide to form carbaminohemoglobin, an unstable compound that, once
again, breaks down—only this time it is carbon dioxide that it
releases, in this case to the surrounding environment rather than to
the cells.

In other species, compounds other than hemoglobin perform a similar
function. For example, some types of annelids, or segmented worms,
carry a green blood protein called chlorocruorin that functions in the
same way as hemoglobin does in humans. And whereas hemoglobin is a
molecule with an iron atom at the center, the blood of lobsters and
other large crustaceans contains hemocyanin, in which copper occupies
the central position. Whatever the substance, the compound it forms
with oxygen and carbon dioxide must be unstable, so that it can break
down easily to release oxygen to the cells or carbon dioxide to the
environment.

Cellular Respiration

Both forms of respiration involve oxygen, but cellular respiration
also involves a type of nutrient—materials that supply energy,
or the materials for forming new tissue. Among the key nutrients are
carbohydrates, naturally occurring compounds that consist of carbon,
hydrogen, and oxygen. Included in the carbohydrate group are sugars,
starches, cellulose, and various other substances.

Glucose is a simple sugar produced in cells by the breakdown of more
complex carbohydrates, including starch, cellulose, and such complex
sugars as sucrose (cane or beet sugar) and fructose (fruit sugar). In
cellular respiration, an organism oxidizes glucose (i.e., combines it
with oxygen) so as to form the energy-rich compound known as adenosine
triphosphate (ATP). ATP, critical to metabolism (the breakdown of
nutrients to provide energy or form new material), is the compound
used by cells to carry out most of their ordinary functions. Among
those functions are the production of new cell parts and chemicals,
the movement of compounds through cells and the body as a whole, and
growth.

In cellular respiration, six molecules of glucose (C
6
H
12
O
6
) react with six molecules of oxygen (O
2
) to form six molecules of carbon dioxide (CO
2
), six molecules of water (H
2
O), and 36 molecules of ATP. This can be represented by the following
chemical equation:

6C
6
H
12
O
6
+ 6 O
2
→ 6 CO
2
+ 6 H
2
O + 36 ATP

The process is much more complicated than this equation makes it
appear: some two dozen separate chemical reactions are involved in the
overall conversion of glucose to carbon dioxide, water, and ATP.

The Mechanics of Breathing

All animals have some mechanism for removing oxygen from the air and
transmitting it into the bloodstream, and this same mechanism
typically is used to expel carbon dioxide from the bloodstream into
the surrounding environment. Types of animal respiration, in order of
complexity, include direct diffusion, diffusion into blood, tracheal
respiration, respiration with gills, and finally, respiration through
lungs. Microbes, fungi, and plants all obtain the oxygen they use for
cellular respiration directly from the environment, meaning that there
are no intermediate organs or bodily chemicals, such as lungs or
blood. More complex organisms, such as sponges, jellyfish, and
terrestrial (land) flatworms, all of which have blood, also breathe
through direct diffusion. The latter term describes an exchange of
oxygen and carbon dioxide directly between an organism, or its
bloodstream, and the surrounding environment.

More complex is the method of diffusion into blood whereby oxygen
passes through a moist layer of cells on the body surface and then

O
NE FORM OF RESPIRATION IS DIFFUSION INTO BLOOD
,
WHEREBY OXYGEN PASSES THROUGH A MOIST LAYER OF CELLS ON THE BODY
SURFACE
,
THEN THROUGH CAPILLARY WALLS AND INTO THE BLOODSTREAM
,
WHERE IT MOVES ON TO TISSUES AND CELLS
. A
MONG THE ORGANISMS THAT RELY ON DIFFUSION INTO BLOOD ARE
ANNELIDS
,
A GROUP THAT INCLUDES LEECHES
. (

through capillary walls (capillaries are small blood vessels that
form a network throughout the body) and into the bloodstream. Once
oxygen is in the blood, it moves throughout the body to different
tissues and cells. Among the organisms that rely on diffusion into
blood are annelids, a group that includes earthworms, various marine
worms, and leeches.

In tracheal respiration air moves through openings in the body surface
called spiracles. It then passes into special breathing tubes called
tracheae that extend into the body. The tracheae divide into many
small branches that are in contact with muscles and organs. In small
insects, air simply moves into the tracheae, while in large insects,
body movements assist tracheal air movement. Insects and terrestrial
arthropods (land-based organisms with external skeletons) use this
method of respiration.

Much more complicated than tracheae, gills are specialized tissues
with many infoldings. Each gill is covered by a thin layer of cells
and filled with blood capillaries. These capillaries take up oxygen
dissolved in water and expel carbon dioxide dissolved in blood. Fish
and other aquatic animals use gills, as did the early ancestors of
humans and other higher animals. A remnant of this chapter from
humans' evolutionary history can be seen in the way that an
embryo breathes in its mother's womb, not by drawing in oxygen
through its lungs but through gill-like mechanisms that disappear as
the embryo develops.

LUNGS.

Lungs are composed of many small chambers or air sacs surrounded by
blood capillaries. Thus, they work with the circulatory system, which
transports oxygen from inhaled air to all tissues of the body and also
transports carbon dioxide from body cells to the lungs to be exhaled.
After air enters the lungs, oxygen moves into the bloodstream through
the walls of these capillaries. It then passes from the lung
capillaries to the different muscles and organs of the body.

Although they are common to amphibians, reptiles, birds, and mammals,
lungs differ enormously throughout the animal kingdom. Frogs, for
instance, have balloon-like lungs that do not have a very large
surface area. By contrast, if the entire surface of an adult male
human's lungs were spread flat, it would cover about 750 sq.
ft. (70 m
2
), approximately the size of a handball court. The reason is that
humans have about 300 million gas-filled alveoli, tiny protrusions
inside the lungs that greatly expand the surface area for gas
exchange.

Birds have specialized lungs that use a mechanism called crosscurrent
exchange, which allows air to flow in one direction only, making for
more efficient oxygen exchange. They have some eight thin-walled air
sacs attached to their lungs, and when they inhale, air passes through
a tube called the bronchus and enters posterior air sacs—that
is, sacs located toward the rear. At the same time, air in the lungs
moves forward to anterior air sacs, or ones located near the
bird's front. When the bird exhales, air from the rear air sacs
moves to the outside environment, while air from the front moves into
the lungs. This efficient system moves air forward through the lungs
when the bird inhales and exhales and makes it possible for birds to
fly at high altitudes, where the air has a low oxygen content.

Humans and other mammals have lungs in which air moves in and out
through the same pathway. This is true even of dolphins and whales,
though they differ from humans in that they do not take in nutrition
through the same opening. In fact, terrestrial mammals, such as the
human, horse, or dog, are some of the only creatures that possess two
large respiratory openings: one purely for breathing and smelling and
the other for the intake of nutrients as well as air (i.e., oxygen in
and carbon dioxide out).